Trigger iRule for raw IP packets

I am trying to create an iRule to fire when a RAW IP packet traverses the F5 LTM. My ultimate goal is to load balance GRE traffic based upon specific attributes of the packet. The way I've been approaching this is as follows:

 

• Create a virtual server pool of my routers, with a type of performance L4, all ports, IP protocol 47.
• Each premises router points their GRE destination to the F5 virtual server IP.
• The F5 DNATs the packet to one of the pool members.

In between I'm trying to use an iRule to achieve persistance based on the inside source IP of the GRE packet. I've tried various conditions such as FLOW_INIT, CLIENT_ACCEPTED. I think the problem is that the F5 is treating the GRE packet as raw IP and being raw IP there really are no "flows" to create unlike TCP\UDP so the iRules are not triggering.